1. Field of the Invention
The present invention relates to a packet switching apparatus with multi-channel and multi-cast switching functions and a packet switching system using the same, and particularly, to an improved packet switching apparatus with multi-channel and multi-cast switching functions and a packet switching system using the same which are capable of using an ATM (Asynchronous Transfer Mode) which is a fixed length switching apparatus as a key element for a B-ISDN, providing a multi-channel switch structure having a physical and logical relationship between input/output ports, whereby it is possible to enhance a cell processing capacity of a switch.
2. Description of the Conventional Art
The key technique for a building-up of the B-ISDN is an ATM design which performs a transmission and exchange of a fixed length packet unit. In particular, the switching system which becomes a key element in designing a network apparatus must be made based on an ATM switching method. So far, many studies have been conducted concerning the ATM switch structure for the above-described purposes. As a result such intensive studies, a part of the technique is actually used in the industry.
The speed of a link which is used based on the building-up of the B-ISDN is variable. In particular, a high speed link is a consideration in the industry. As the basic speed of the initial stage of the B-ISDN, there are an STM (Synchronous Transfer Mode) -1 class 155 Mbps and an STM-4 class 622 Mbps. Recently, at the advent of high speed optical transport systems, bit rates of 2.5 Gbps, 10 Gbps, and even 100 Gbps are possible. For the reasons of the high speed and economic service network, in a DAVIC (Digital Audio and Video International Council), the ATM system requires a slower speed link as 25 Mbps and 51 Mbps. Therefore, the ATM switching system requires a switching method which is capable of effectively using various channel speed.
However, most conventional switching methods are used as a single channel switching method which has one-to-one concept in an input/output port of a switch network. Hence, the single channel switching method means that the output port physically and logically one link. Therefore, a bandwidth allocation and a routing path determination in a switch network are independently performed with respect to each port. Therefore, to support a virtual connection of nXV bps in a switching network with basic port speed of V bps, it is necessary to employ a complex multiplexing/demultiplexing scheme. Such a scheme, which enforces cells from a single virtual connection to traverse over the same physical input/output port, is essential in order not to separate cells from the virtual connection. For the above-described reasons, the construction of the system is complex, and the system is costly for the implementation thereof.
Differently from the above-described single channel switching method, the multi-channel switching method is configured in a multiple-to-multiple input/output port method. In addition, a plurality of input/output links is configured as one group, so that it is possible to logically perform a switching service as one port.
An arrived cell may exit from any output port belonging to a single logical group in the switch, and this improves a chance of exiting from the switch in a given time slot, based on a well known queueing discipline called the economies of scale. In addition, since it is possible to share the maximum total band width of multiple links of one logical port, it is possible to effectively use an effective bandwidth. In addition, the burst characteristic of an ATM traffic is adapted, for thus improving a cell loss characteristic.
Therefore, it is possible to concurrently use input/output links which need different speeds by using one switch network. At this time, since a time division multiplexor/demultiplexor having a simple function is used, the construction of the system is simple, and the cost of the system is low.
For the above-described necessities, a switching method which provided a few multi-channel switching functions are disclosed. However, there is a problem in providing the multi-channel switching, so that a cell sequence integrity method which is an important operational characteristic is not clear, for thus degrading the efficiency of the system.
In the switching system, integrating the sequence of the cells inputted is one of the important characteristics. Hence, if the sequence of the cells is not integrated in the switch network, a complex function is additionally necessary for the integrity of the cell sequence. In addition, in the end point of the service, a function for controlling the sequence of the cells is additionally necessary.
In particular, in the multi-channel switching method, the above-described function is more important. Namely, the sequence of the cells inputted through one input link and the input sequence of the cells of one virtual connection which cells are distributed to a plurality of input links must be maintained when outputting the same through the grouped links consisting of a plurality of output links.
However, in the conventional multi-channel switching method, the above-described sequences of the cells are not provided to the switch network, and the sequence information is added to a payload of the cell, and then is transferred to the end point of the service, for thus adjusting the sequence of the cells by using a buffer mechanism at the stage of the service end point.
Therefore, both end points of the service must have a specific protocol for the cell sequence integrity, and the identification of the protocol must be maintained. In addition, since the sequence information is added to the payload of the cell, the transmission efficiency of the cell is degraded.
The multicasting feature requires certain cells to be copied inside the switch. This cell copying function is traditionally done by a separate copy network, which along with the routing network, forms the overall space division switch. Namely, the cells of a desired number of ports is copied, and the paths are provided with respect the copied cells through the routing network.
For the above-described operations, the cells are copied to a plurality of output ports by using the information reacted to the amount of the copies, and the routing network must assign the path in accordance with the output port information, to which the cells with respect to each cell which is distributed to multiple ports must finally outputted. Here, the problems are that two networks of the copy network and the routing network are needed for the multi-casting service, and it is difficult, if not impossible, to avoid the placement of a large size look up table between routing and copy networks.
Namely, in the copy network, the cell copy is performed by only a numerical information. When the copied cells are outputted to the output port of the copy network, a fixed output port is not assigned with respect to the cell of one virtual connection. Namely, the copied cells are outputted to a temporary port at every cell time.
Therefore, it is necessary to configure the table for determining that a corresponding cell is outputted which output port of the routing network by using an inherent information contained in the cell outputted, such as a connection number which is assigned to one value of the system, namely, a connection identification number, etc. Namely, since it is impossible to detect the copied cell is outputted to which output port of the copy network, the routing table must be configured in each output port of the copy network, and the identical routing path information must be duplicatingly provided in each table.
With the above-described operation and configuration, the entire switching system is complex. In particular, if there are many virtual paths for supporting by the input link, the capacity of the table must be large. As the speed of the system input/output link is made high, since the number of virtual connections for supporting is exponentially increased, the system is costly, and there is a problem in implementing a high speed table access.
The capacity extension of the switch network is conventionally obtained by extending a unit switch of a proper capacity to a multi-stage interconnection network. As the type of the multi-stage interconnection network, there are Banyan network, Clos network, etc. However, such the multi-stage interconnection network uses switch units based on a single channel switching paradigm. Thus, even if there are multiple physical links connecting two switch units in two successive stages, all cells belonging to the same virtual connection must utilize a single link out of these multiple links over the entire duration of connection.
When the virtual connection is being established in a switch, one specific link must be selected for each inter-stage connection. This implies that all links in the inter-stage connection must be checked individually for their available capacity. In this case, if there is not enough available capacity in any of the links checked, the connection cannot be established even if there is sufficient capacity available scattered over multiple links.